Main Vent Clogged

My main vent stack is clogged and I would like some advice on cleaning it. I have an old, 1900's home I recently purchased. Upstairs bathroom has toilet which overflows occasionally. Toilet does drain and shower and sink works well but I do hear gurgling in sink when toilet is flushed.

I know this is probably vent stack clogged so I went on roof, tall two story home and don't want to be up there, and tried sticking a 8' piece of conduit down it tied to rope. End of conduit when pulled back up has leaf litter stuck in it, tried bashing my way through with conduit to no avail.

Next tried drain cleaning bladder with water hose and promptly filled up drain vent with water going nowhere but out the top and through fittings. I think water did go through some of the fitting in attic where one piece of cast iron and another connect but cannot be sure, it could have leaked down from through roof onto pipe fitting.

Called plumber when water did not appreciably move down vent stack and he said its a clogged vent pipe. He wants to cut vent pipe in second story bathroom and add cleanout T as well as snake vent pipe from there. Vent pipe goes straight down from ceiling into second story bathroom where at floor 90 degree elbows move pipe over a foot or so into floor space where I assume other T's and such connect toilet, sink and shower.

Plumber said clogged is most likely at 90 elbows which I agree but am wondering if other, less work and lower cost solution is doable. I can get at most of pipe in bathroom but taking out some of shower surround may be needed.

Can i get at clog from attic and work there with adding cleanout more easily than in bathroom. There is no obstructions in attic and I would feel better working there than on roof to try and clear clog manually.

Can I use some kind of chemical cleaner and possibly clear clog that way and then just keep an eye on vent in the future.

Any ideas would be helpful.

Forgot to mention, plumber would not go on roof, others are wanting extra for rooftop work so I really don't want to work that route if possible.